1. Field of the Invention
This invention relates generally to the field of detection of a change in reflected light in the presence of ambient light, and more particularly to the detection of the application of liquid to a reflecting surface or reaction chamber having a highly reflecting surface using modulated light.
2. Description of the Related Art
In various biomedical applications, it is very desirable to be able to detect when the level of reflected light from a surface has changed. This is particularly useful when it is desired to detect the application of liquid to a reaction chamber such as that described by Oberhardt in U.S. Pat. Nos. 4,849,340 and 5,110,727.
In order to accurately measure a time duration between the start of a biochemical reaction and its endpoint in the reaction chamber, the beginning time of the reaction must be accurately known. In the types of reactions taught by Oberhardt, the start time occurs when a drop of blood or serum is placed into the chamber. This application of liquid causes the chamber to take on a different index of refraction or a different absorption. Either of these, causes a reflected beam of light to change amplitude.
Previous methods of detecting this change simply looked for an increase or decrease in reflected light level whose absolute value was greater than a certain fixed threshold. These methods are very susceptible to false triggering from changes in ambient light reaching the detector and from vibration or movement of the chamber. For example, flashes of light, or changes in shadows entering the detector, could cause the system to false trigger and start the reaction timing prematurely. Also, when the reaction chamber is mounted on a card of some sort for backing, and the card is slightly moved with respect to the light source or detector, a false trigger can occur. Such motion can occur when a blood dispenser such as DIFF-SAFE (R) made by Alpha Scientific Corp. of Southeastern, Pennsylvania. inserted into a Vacutainer (R) made by Becton Dickinson of Rutherford, N.J. is touched to the surface of the test card prior to dispensing a drop of blood or serum.
It would be extremely desirable to have a detection method that is very sensitive to changes in reflected light but very insensitive to changes in ambient light and insensitive to vibration or transient motion of the reflective surface.